Friday, March 3, 2017

What are the wettest and driest months of the year? Using published normal values, we can answer that question. These maps were generated using the 1981-2010 NCEI climate normals for monthly precipitation within the U.S. In Canada and Mexico (and the rest of the globe [not shown]), the GHCN v.2 monthly precipitation was used.

To standardize the months due to differences in the number of days, I used an average daily precipitation value. In a few instances, this will cause discrepancies. For example, if February averages 3.00" of precipitation and March averages 3.10", I show February as having more precipitation – since their per-day value is higher.

On the mapping side of things, how do you interpolate a month between two stations where one is wettest in December and the other is wettest in January. If you average the month numbers (12 and 1), you end up with June being the wettest month! Obviously this is not correct. I used trigonometric functions to compute these polar-coordinate averages. Still, there are boundary issues when nearby stations have vastly different wettest or driest months.

There are four sets of maps.

1) Wettest / Driest month of the year (color shaded)

2) Wettest / Driest month of the year (station dots)

3) Wettest / Driest season of the year (color shaded)

4) Wettest / Driest season of the year (station dots)

Note: Seasons are based on calendar months (e.g., winter = December through February).

Wettest / Driest month of the year (color shaded)

Figure 1. Wettest month of the year based on climate normals (color shaded).

Figure 2. Driest month of the year based on climate normals (color shaded).

Wettest / Driest month of the year (station dots)

Figure 3. Wettest month of the year based on climate normals (station dots).

Figure 4. Driest month of the year based on climate normals (station dots).

Wettest / Driest season of the year (color shaded)

Figure 5. Wettest season of the year based on climate normals (color shaded).

Figure 6. Driest season of the year based on climate normals (color shaded).

Wettest / Driest season of the year (station dots)

Figure 7. Wettest season of the year based on climate normals (station dots).

Figure 8. Driest season of the year based on climate normals (station dots).